Objective: To carry out further studies on the deoxynucleoside kinases of calf thymus and Lactobacillus acidophilus with intent to answer the following questions: 1) In each organism does a single protein catalyze the phosphorylation of deoxycytidine, deoxyguanosine, deoxyadenosine and arabinosyl cytosine? 2) By what mechanism(s) do the several end-product nucleotides regulate the phosphorylation of each of the deoxynucleosides; are there separate binding sites for each nucleoside and/or end product? 3) Why isn't the homotropic cooperativity produced with the thymus enzyme by dCyd seen with the other substrates? Approach: The purified enzymes will be subjective to amino acid, end-group, sub-unit and kinetic analysis. Binding studies will determine the number of sites for each nucleoside and nucleotide effector. Antigenic purity will provide an additional criterion of homogeneity. Substrate analogues and selective chemical attack will be used in attempts to modify or inactivate one or more of the homotropic or heterotropic binding sites.